Known aldehyde tests include the Sawicki and Purpald reactions. Both these known reactions, and especially the latter, are good for assaying formaldehyde. They are less sensitive, to varying degrees, to other aldehydes. Moreover, the Purpald reaction requires alkali, and may therefore be experimentally-undesirable.
Known biocides include cationic surfactants, such as tributyltetradecylphosphonium chloride (e.g. as sold under trade mark name Bellacide 350), glutaraldehyde and methylene-bisthiocyanate (MBTC). They are used in cooling towers, in order to control organism such as Legionella, in paper-making and in other aqueous environments. It is an object behind this invention to monitor the level of such compounds in the water. The invention is based, at least in part, on the realisation that MBTC releases formaldehyde under alkaline conditions such as those that prevail in cooling towers. The Purpald reaction is clearly unsuitable in these circumstances, since it cannot distinguish between MBTC and any free formaldehyde with which such a compound may be in equilibrium or with any formaldehyde from another source.